Small molecule Lewis acid/base complexes of organoboranes and amines are known. They can be formed by the reaction of a triorganoborane (a Lewis acid) and an amine (a Lewis base). Amines that form the most stable complexes with triorganoboranes include primary amines and some secondary amines. Tertiary amines, sterically hindered secondary amines, and amines in which the nitrogen atom lone electron pair is delocalized (and thus not available to form a strong dative bond with the boron atom via the boron empty p orbital) form less stable complexes with organoboranes. The complexes have the general structureR3B←NR′3 wherein each R is independently an alkyl group, a cycloalkyl group, or an aralkyl group, and each R1 is independently H, an alkyl group, or a cycloalkyl group.
The triorganoborane-amine complexes can be “decomplexed” by compounds that react with amines, thereby liberating free triorganoborane. This reaction is typically irreversible such that the amine is then no longer available to complex the triorganoborane. The free triorganoborane can react with oxygen to generate several free-radical species, some of which are known to initiate radical polymerization of unsaturated monomers, such as acrylates.
While such conventional triorganoborane-amine complexes are more stable than the free triorganoborane with respect to oxidation by atmospheric oxygen, there is still a need for even greater stability. Furthermore, such conventional triorganoborane-amine complexes are liquids often kept under an inert atmosphere, and thus are not easily stored, transported, or handled.